As the industrial field has diversified and advanced in recent years, electronic devices such as audio-visual devices, servers and personal computers have been dramatically reduced in size and weight, increased in memory capacity, and increased in information processing speed. Such electronic devices employ various rotating devices, for example, rotating devices for driving magnetic disks and optical discs such as FD, MO, zip, mini disc, compact disc (CD), DVD and hard disk. The reduction in size and weight of and the increase in memory capacity and information processing speed of these electronic devices are greatly attributed to improvement in bearings essential to the rotating devices. In particular, a fluid dynamic bearing, which is made up of a sleeve member and a shaft member facing each other via a lubricant, is not only suitable to reduce the size and weight of an electronic device but also excellent in silence, economical efficiency and the like, because the fluid dynamic bearing does not have a ball bearing. Thanks to these characteristics, the fluid dynamic bearing has been increasingly used in servers, personal computers, audio devices, visual devices, car navigation systems and the like.
Examples of properties which a lubricant should have include (i) excellent energy-saving (i.e., low viscosity), (ii) excellent heat resistance (i.e., oxidation resistance, volatility resistance, and a small change in viscosity due to an increase in temperature), (iii) excellent fluidity at low temperature (excellent low-temperature fluidity), and like properties, in addition to basic properties (e.g., lubricity, stability against deterioration (lifetime), property of preventing sludge from being generated, anti-wearing property, anti-corrosion property and like properties).
For example, in a case where, in electronic devices such as audio-visual devices, servers and personal computers, a bearing is rotated at high speed for high-speed processing of large-volume information, load on the bearing increases, thereby remarkably increasing an internal temperature of the bearing. Because of this, great importance is placed on heat resistance of properties which a bearing lubricant should have.
These electronic devices have been increasingly required not only to be capable of high-speed processing of large-volume information but also to be further reduced in size. These electronic devices can be reduced in size by increasing the lifetime of or reducing the capacity of a battery contained in these electronic devices. These electronic devices therefore have been still strongly required to be more energy-saving. In order for the electronic devices to be more energy-saving, one way is to use a lubricant having a low viscosity as a bearing lubricant.
Moreover, as the foregoing information processing devices have been popularized, there have been an increasing number of cases where they are used under harsher environments. In particular, car navigation systems etc. which are mounted on a vehicle are required, considering the environment in which the vehicle is used, to be resistant to a wide range of temperatures including those of cold climates and hot sunlight. Therefore, in a case where a bearing lubricant is used in on-vehicle devices, it is required to be usable without any problems in a wide range of temperatures, for example, from −20° C. to 120° C. To this end, there is a demand for a lubricant that keeps a low viscosity even at low temperatures and suffers little evaporation loss even at high temperatures.
In a further case where a lubricant is used as a lubrication oil of a freezer (freezer oil), the lubricant is required to be (iv) excellent in compatibility with or solubility to a coolant, in addition to the above properties (i) through (iii).
Synthetic hydrocarbon oils and organic acid esters are used as traditional lubricants. For example, Patent Literature 1 discloses a hindered ester synthetic lubrication base oil.
Patent Literature 2 discloses a freezer working fluid composition that contains a specific ester whose iodine number is not more than 1 (1 g/100 g).
Patent Literature 3 discloses a base oil made of (i) a specific composite ester and (ii) dicarboxylic acid ester of adipic acid.
Patent Literature 4 discloses a lubrication base oil composition that contains a specific dibasic acid ester compound and polyester.
Patent Literature 5 discloses that a specific alkoxy ester is suitable for a lubricant.